1. Field of the Invention
The present invention relates to a flow quantity measuring apparatus.
2. Description of Related Art
For instance, Japanese Patent JP3675721B2 and Japanese Unexamined Patent Publication JP2006-258675A teach a flow quantity measuring apparatus, which includes a thermal air flow meter. The thermal air flow meter includes a heat generating resistor and temperature sensitive resistors, a resistance value of which changes depending on the temperature, to sense the flow quantity of the air.
Specifically, the thermal air flow meter includes a sensor chip, a heater resistor (heat generating resistor), two upstream-side temperature sensor resistors (upstream-side temperature sensitive resistors), two downstream-side temperature sensor resistors (downstream-side temperature sensitive resistors) and an intake air temperature sensor resistor. The sensor chip includes a silicon circuit board, on which a membrane (thin wall portion) and a group of electrode pads are formed. The heater resistor generates heat upon receiving a heating electric current. The upstream-side temperature sensor resistors are placed on an upstream side of the heater resistor in the flow direction of the air. The downstream-side temperature sensor resistors are placed on a downstream side of the heater resistor. The intake air temperature sensor resistor senses the ambient temperature (temperature of the intake air) around the intake air temperature sensor resistor.
The heater resistor is placed on the membrane of the sensor chip. Each of the upstream-side temperature sensor resistors and the downstream-side temperature sensor resistors is placed at a corresponding location on the membrane of the sensor chip where each of the upstream-side temperature sensor resistors and the downstream-side temperature sensor resistors is influenced by the heat generated from the heater resistor. The intake air temperature sensor is placed on a corresponding location on the sensor chip where the intake air temperature sensor is not influenced by the heat generated from the heater resistor.
The membrane of the sensor chip has a film thickness, which is smaller than that of the other portion of the sensor chip, so that a heat capacity of the membrane is small, and thereby it is possible to obtain a quick thermal response in response to a change in the flow quantity of the air.
A flow quantity sensing circuit (operational amplifier), which senses the air flow quantity, includes a bridge circuit that has a first series circuit and a second series circuit connected with each other in parallel. In the first series circuit, the corresponding upstream-side temperature sensor resistor and the corresponding downstream-side temperature sensor resistor (first resistor) are connected with each other in series. In the second series circuit, the corresponding downstream-side temperature sensor resistor and the corresponding upstream-side temperature sensor resistor (second resistor) are connected with each other in series. A resistance value of each temperature sensor resistor changes depending on the temperature, so that it is possible to obtain a voltage signal, which corresponds to the air flow quantity, through sensing of a change in the resistance value. Specifically, the flow quantity sensing circuit is constructed such that a midpoint electric potential difference of the bridge circuit is converted into a corresponding voltage that is, in turn, outputted as an air flow quantity voltage signal from the flow quantity sensing circuit to an engine control unit (ECU).
A first connection, which is between the corresponding upstream-side temperature sensor resistor and the corresponding downstream-side temperature sensor resistor, and a second connection, which is between the corresponding upstream-side temperature sensor resistor and the corresponding downstream-side temperature sensor resistor, are placed at a location outside of the sensor chip (e.g., placed in conductive lines on a circuit chip). That is, conductive lines, through which the midpoint electric potential difference is outputted from the bridge circuit, are branched from the first connection and the second connection, respectively, of the circuit chip.
Furthermore, the drive circuit (an operational amplifier and a transistor) of the heater resistor includes a bridge circuit, which has a third series circuit and a fourth series circuit. In the third series circuit, the heater resistor and a third resistor are connected with each other in series. In the fourth series circuit, the intake air temperature sensor resistor and a fourth resistor are connected with each other in series. The heating electric current, which flows through the heater resistor, is variably controlled by the drive circuit of the heater resistor such that the midpoint electric potential difference of the bridge circuit becomes 0 (zero).
Furthermore, a third connection, which is between the heater resistor and the third resistor, and a fourth connection, which is between the intake air temperature sensor resistor and the fourth resistor, are placed at a location outside of the sensor chip (e.g., placed in conductive lines on the circuit chip). That is, conductive lines, through which the midpoint electric potential difference is outputted from the bridge circuit, are branched from the third connection and the fourth connection, respectively, of the circuit chip.
Japanese Unexamined Patent Publication JP2009-264741A and Japanese Unexamined Patent Publication JP2009-270930A teach a flow quantity measuring apparatus, in which electrode pads on a sensor chip and electrode pads on a circuit chip are connected through bonding wires, and portions of resistor conductive lines of each temperature sensor resistor, the electrode pads on the sensor chip, the electrode pads on the circuit chip and the bonding wires are covered with molded epoxy resin (encapsulating resin). Furthermore, portions of resistor conductive lines of the heater resistor and the intake air temperature sensor resistor, the electrode pads on the sensor chip the electrode pads on the circuit chip, the bonding wires and the conductive lines on the circuit chip are also covered with the molded epoxy resin (encapsulating resin).
However, in the flow quantity measuring apparatus of Japanese Patent JP3675721B2 and Japanese Unexamined Patent Publication JP2006-258675A, each resistor conductive line (the electrode pad on the sensor chip, the electrode pad on the circuit chip, the bonding wire and the connecting conductive line on the circuit chip), which extends from the corresponding temperature sensor resistor to the corresponding conductive line for outputting the midpoint electric potential difference of the bridge circuit, is very long. Therefore, an unnecessary conductive line resistance is applied on the bridge circuit, so that the detection sensitivity for detecting, i.e., sensing the air flow quantity is disadvantageously deteriorated.
Similarly, each resistor conductive line (the electrode pad on the sensor chip, the electrode pad on the circuit chip, the bonding wire and the connecting conductive line on the circuit chip), which extends from a corresponding one of the heater resistor and the intake air temperature sensor resistor to the corresponding conductive line for outputting the midpoint electric potential difference of the bridge circuit, is very long. Therefore, an unnecessary conductive line resistance is applied on the bridge circuit, so that the control sensitivity (accuracy) for controlling the heating temperature (heater temperature) of the heater resistor is disadvantageously deteriorated.
Furthermore, in the flow quantity measuring apparatus of Japanese Unexamined Patent Publication JP2009-264741A and Japanese Unexamined Patent Publication JP2009-270930A, surfaces of the resistor conductive lines, which are located adjacent to the electrode pads on the sensor chip, and the connecting conductive lines, which are located on the circuit chip, are covered with the molded resin, which has a different coefficient of linear expansion that is different from that of the resistor conducive lines and the connecting conductive lines. Therefore, when a stress is applied to the resistance conductive lines, which are located on the sensor chip, and the connecting conductive lines, which are located on the circuit chip, the conductive line resistance value and/or the temperature coefficient of resistance will be disadvantageously changed, i.e., fluctuated.
Here, it is conceivable to increase the temperature of the heater resistor from the temperature of the heater resistor of the current product to improve the detection sensitivity for detecting, i.e., sensing the air flow quantity. However, when the temperature of the heater resistor is increased, the heating electric current, which flows through the heater resistor, is also increased, thereby disadvantageously resulting in an increase in the electric consumption. Also, when the temperature of the heater resistor is increased, the thermal degradation of the heater resistor is promoted. Therefore, the durability (lifetime) of the heater resistor is disadvantageously deteriorated.
Japanese Unexamined Patent Publication JP2003-315130A (corresponding to U.S.2003/0182999A1) teaches a flow quantity measuring apparatus, in which all of resistors (a heater resistor, fixed resistors and an intake air temperature sensor resistor) are formed such that an insulation film (lower film) is formed on a common silicon circuit board (silicon chip), and a resistor film (thin film) of, for example, platinum is formed in a predetermined pattern on the insulation film through a vacuum vapor deposition process. In this way, a film thickness of the resistor film is stabilized, and thereby variations in the resistance values of all of the resistors of the bridge circuit are limited.
Furthermore, Japanese Unexamined Patent Publication JP2003-315130A (corresponding to U.S.2003/0182999A1) also teaches that the bridge circuit, which is used in the flow quantity sensing circuit, includes first and second upstream-side temperature sensor resistors and first and second resistors. Furthermore, a first connection, a first lead conductive line, a second connection and a second lead conductive line are formed on the insulation film of the silicon chip. The first upstream-side temperature sensor resistor and the first resistor are connected with each other at the first connection. The midpoint electric potential is outputted from the first connection to a first electrode pad through the first lead conductive line. The second upstream-side temperature sensor resistor and the second resistor are connected with each other at the second connection. The midpoint electric potential is outputted from the second connection to a second electrode pad through the second lead conductive line.
In the flow quantity measuring apparatus of Japanese Unexamined Patent Publication JP2003-315130A (corresponding to U.S.2003/0182999A1), output points, through which the midpoint electric potentials are outputted, are not provided in a midpoint of a first connecting conductive line, which connects between the first upstream-side temperature sensor resistor and the first resistor, and a midpoint of a second connecting conductive line, which connects between the second-upstream side temperature sensor resistor and the second resistor. That is, the first and second connections are not formed in the midpoints, respectively, of the first and second connecting conductive lines.
Therefore, like in the cases of JP3675721B2, JP2006-258675A, JP2009-264741A, JP2009-270930A and JP2003-315130A (corresponding to U.S.2003/0182999A1) where the bridge circuit, which is used in the flow quantity sensing circuit, is formed with two pairs of temperature sensor resistors (temperature sensor resistors connected in series) that need to have pair characteristics (identical characteristics) with respect to the temperature characteristics of the resistance value, the required pair characteristics with respect to the temperature characteristics of the resistance value cannot be maintained in the temperature sensor resistors of each pair. Thereby, in such a case, the detection sensitivity for detecting, i.e., sensing the air flow quantity may possibly be deteriorated.